1. Field of the Invention
The present invention relates to a monitoring electrode for monitoring dorsal cochlear nucleus action potentials (DNAPs), and a monitoring device for monitoring the DNAPs using the monitoring electrode for monitoring the DNAPs.
2. Description of the Related Art
FIG. 6 is a half cross sectional diagram of a head explaining a structure of a right ear. As shown in FIG. 6, a human ear comprises an auricle which is a shell-shaped protrusion surrounding an external acoustic meatus located at both sides of the head, an external auditory meatus, an eardrum, an ear ossicle of a middle ear, a cochlea D of an inner ear, and the other organ connecting to an auditory nerve 4. Further, a dorsal cochlear nucleus G is located at a position covered by a cerebellum F, at a brainstem E in the center of the brain.
FIGS. 7A and 7B are magnified cross sectional views showing a position of nerves in a right internal auditory canal. FIG. 7A is a magnified cross sectional view showing an early stage of an acoustic neuroma. FIG. 7B is a magnified cross sectional view typically showing a late stage of the acoustic neuroma. As shown in FIG. 7A, an auditory nerve 4 includes three types of nerves: a superior vestibular nerve 4a conducting a balance sense, an inferior vestibular nerve 4b, and a cochlear nerve 4c conveying an auditory sense. Adding a facial nerve 5 controlling facial movement, four nerves run in the internal auditory canal.
The acoustic neuroma is a benign tumor, which is frequently found and diagnosed in the field of otolaryngology and neurological surgery by a sign of a deterioration of hearing for an ear at one side. As shown in FIG. 7B, the acoustic neuroma occurs at the superior vestibular nerve 4a or the inferior vestibular nerve 4b, and gradually proliferates toward the brainstem. FIG. 7B is a diagram showing a state that the acoustic neuroma occurs at the superior vestibular nerve 4a. Inside the internal auditory canal, the superior vestibular nerve 4a is located closely adjacent to a facial nerve 5. When the acoustic neuroma is growing, the inferior vestibular nerve 4b is deformed pressed by the acoustic neuroma. As a result, a patient feels a trembling sense and a disorder of a balance sense. Further, symptoms such as a deterioration of hearing caused by the cochlear nerve 4b under pressure, a ringing in ears, and a paralysis of the facial nerve 5, are liable to be shown.
Moreover, if the acoustic neuroma hypertrophies, a trigeminal nerve and the brainstem are pressed to cause a headache and a failing of eyesight, and sometimes this leads to a fatal situation. If it is the case, surgery for removing the acoustic neuroma is needed.
In the surgery, it is important not only to remove the acoustic neuroma as maximum as possible, but also to preserve a function of the cochlear nerve 4c which touches the acoustic neuroma and already suffers from a deformation disorder. Therefore, in the conventional surgery, in order to continuously monitor an auditory (nerve) function, an auditory brainstem response (ABR) needle electrode 7 using a subdermal needle A (see FIG. 9) and a cochlear nerve action potential (CNAP) plate electrode 8 (see FIGS. 10A to 10C) using a plate C, are used as shown in FIG. 6. The ABR needle electrode 7 is attached to a scalp at a rear portion of an ear to be used for auditory brainstem responses (ABRs). The ABR needle electrode 7 monitors the auditory brainstem responses (ABRs) by a scalp needle electrode. Potentials collected at the place where the electrode is attached are displayed on a monitor screen.
For another type of an electrode, the CNAP plate electrode 8 is held on the auditory nerves 4 between the cochlear D and the brainstem E, or inserted into a spot of the auditory nerve 4. A potential detected at this spot is displayed on the monitor screen. Then, by monitoring a waveform displayed on the screen, the surgery is performed by taking care not to change the waveform.
FIG. 9 is a plan view showing an ABR needle electrode using a conventional subdermal needle. As shown in FIG. 9, at the left edge of the ABR needle electrode 7, the subdermal needle with a φ0.45 mm diameter is provided at a 15 mm length. The electrode 7 is divided to two lead wires 7e of a 20 cm length, through a tube cover 7b, a lead wire 7c of a 60 cm length, and a tube cover 7d. At the right end of the electrode 7, molded pin tips 7f are connected. A total length of the ABR needle electrode 7 is about 1 m.
FIGS. 10A to 10C show a conventional CNAP plate electrode. FIG. 10A is a magnified plan view. FIG. 10B is a magnified front view. FIG. 10C is a magnified cross sectional view. As shown in FIGS. 10A to 10C, an electrode part 8a of the CNAP plate electrode 8 has a disk shape, of which diameter d is about 2.5 mm. A protrusion part 8b is formed for a gripping part to be used during an insertion. A pin tip 8d is connected to the protrusion part 8b with a lead wire 8c of about 1 m. As shown in FIG. 10C, a material of the electrode 8a is a platinum plate, of which thickness is 0.1 mm. At the rear face (upper face) of the electrode 8, a lead wire 8c is soldered (8e). The upper face of the electrode part 8 is molded by a rapid 8f made of resin in order to reduce background noises.
FIG. 11 is a diagram showing a state that the CNAP plate electrode 8 is held. As shown in FIG. 11, the CNAP plate electrode 8 is held on the auditory nerve 4.
FIGS. 8A to 8C show graphs of potentials measured by conventional electrodes, and a graph of potentials measured by an electrode of the present invention. FIGS. 8A and 8B show graphs of conventional monitoring images measured by the ABR needle electrode and the CNAP plate electrode. FIG. 8C shows a graph of a monitoring image measured by the electrode of the present invention.
As shown in FIG. 8A, the ABR needle electrode is a conventional tool, which is a scalp needle electrode. By using the electrode, auditory brainstem evoked potentials are measured, and the potentials are displayed on a monitor screen as a graph of waveforms. That is, a click sound entered into ears is displayed as a brainwave. The ABR needle electrode can be stably held on a scalp. However, the waveform contains many noises due to a long distance from the brainstem, and the recorded waveform of the auditory evoked potentials is unstable. Therefore, 500 times adding and averaging frequency is needed in order to obtain a reproducible waveform. As a result, 30 seconds are needed to obtain one waveform. Herein, an extension of a latent period of ABR I to V waves is observed in FIG. 8A. By keeping a change (delay) of the latent period of the ABR I to V waves in a predetermined range (within 0.8 ms) during an operation, it is known that cochlear nerve function can be effectively maintained. However, because of unstable monitoring waveforms and too long monitoring time, the ABR needle electrode method is not a satisfied method as a real time monitoring method.
As shown in FIG. 8B, a graph of waveforms measured by the CNAP plate electrode is obtained by measuring the cochlear nerve action potential (CNAP) responses by a method in which the CNAP plate electrode 8 is held on the auditory nerve 4 shown in FIG. 11. The waveforms are recorded by amplifying waveforms corresponding to an ABR II wave, III wave and IV wave. The waveforms do not contain many noises because the electrode is held close to the brainstem, and are obtained as large waveforms having 1 to 2 μV maximum for II to IV waves. About 200 times adding and averaging frequency is needed so as to obtain a reproducible waveform and 12 seconds are needed to obtain one waveform. As shown in FIG. 8B, initially obtained high amplitude waves II to IV are lost after completion of total extirpation. Here, the most critical problem is that the electrode can not be stably held. It is suggested that the method using the CNAP plate electrode 8 becomes a more accurate monitoring method than the ABR needle electrode method, if the CNAP plate electrode can be held stably. Therefore, a drawback of the CNAP plate electrode method resides in a stability of waveforms when it is used in each operation.
As mentioned above, the ABR needle electrode 7 held on a scalp provides small waveforms due to a potential attenuation caused by a long distance from the brainstem E. Further, many background noises which occur at a lead wire (electric cord) of the ABR needle electrode 7 are mixed in the waveforms. Thus, a problem of the ABR needle electrode is that it is difficult to analyze the waveforms.
Further, another type of the electrode, the CNAP plate electrode 8, has problems such as extreme instability for attaching the electrode, difficulty in stable fixing, dropping, and shifting from a measuring position, even if it is held on thread shaped nerves with about 2 mm diameter, or is held between the nerves.
For these reasons mentioned above, a need exists for a monitoring electrode and a monitoring device which can easily and surely measure waveforms with less noises and have a superior performance in accuracy, adherent property, comprehensiveness, recording sensitivity, and recording specificity, than conventional monitoring methods such as the ABR electrode method for monitoring auditory evoked potentials from a skin and the CNAP electrode method for monitoring the potentials just on nerves.